Known piezoelectric fuel injectors typically employ piezoelectric actuators made from a stack of a plurality of piezoelectric ceramic discs or plates each connected to an electrode for electrically charging and discharging the stack. The actuator is mechanically arranged for opening and closing an injector valve having a valve needle to meter fuel injected into the engine. In some injectors, the piezoelectric actuator is located in a chamber containing fuel—usually diesel, biodiesel or gasoline—pressurised up to 2000 bar. An example of a piezoelectric fuel injector is disclosed in the applicant's U.S. Pat. No. 6,520,423, which utilises a hydraulic coupling arrangement to lift the valve needle off the valve seat to enable opening of the injector in response to longitudinal expansion of the piezoelectric actuator. Another example of such a fuel injector is described in EP0995901.
FIG. 1 is a perspective view of a known design of piezoelectric actuator 2. The actuator is formed from a stack of a plurality of piezoelectric layers or elements 4 that are separated by a plurality of internal electrodes 6, 8. FIG. 1 is illustrative only and in practice the actuator 2 would include a greater number of layers and electrodes (typically in the order of hundreds) than those shown and with a much smaller spacing. The internal electrodes 6, 8 are divided into two groups: a positive group of electrodes (only two of which are identified at 6) and a negative group of electrodes (only two of which are identified at 8). The positive group of electrodes 6 are interdigitated with the negative group of electrodes 8, with the electrodes of the positive group connecting with a positive external electrode 10 of the actuator 2 and the negative group of electrodes connecting with a negative external electrode (not shown) on the opposite side of the actuator 2 to the positive external electrode 10.
The positive and negative external electrodes receive an applied voltage, in use, that produces an intermittent electric field between adjacent interdigitated internal electrodes 6, 8 that rapidly varies with respect to its strength. Varying the applied field causes the actuator 2 to extend and contract along the direction of the applied field. Typically, the piezoelectric material from which the elements 4 are formed is a ferroelectric material such as lead zirconate titanate, which is known by those skilled in the art as PZT. The actuator construction results in the presence of active regions between internal electrodes of opposite polarity. In use, when a voltage is applied across the external electrodes, the active regions are caused to expand resulting in an extension of the longitudinal axis of the actuator 2.
The high electrical field applied to the elements causes a risk of electrical shorting between the side edges of the internal electrodes of opposite polarity. To prevent such shortening, the exposed electrode faces of the piezoelectric multilayer element are preferably covered with a passivation material exhibiting high dielectric strength.
Passivation of the piezoelectric element guards against electrical shorting across the surface of the actuator as long as the actuator is operated in a dry and fuel free environment.